Corona-discharge electrodes and collecting electrodes are the most important components of a dust-collecting electrostatic precipitator and are shaped and arranged so as to match each other.
In most cases, the collecting electrodes are grounded and the corona-discharge electrodes are connected to a high-voltage source.
The dust particles to be collected are ionized by electrons discharged by the corona-discharge electrodes and are deflected out of the gas stream in the electrostatic field that is established between the corona-discharge electrodes and the collecting electrodes. The dust particles are finally collected on the collecting electrodes.
Particularly the collecting electrodes but also the corona discharge electrodes are shaken in certain intervals of time by rapping blows so that the collected dust is detached and falls down into dust-collecting bins. The effectiveness of a dust-collecting electrostatic precipitator depends on the adoption of the corona discharge electrodes and collecting electrodes for their various functions.
Many corona-discharge electrodes consist of wires or strips which are tensioned in a frame and provided with pointed tips. Associated collecting electrodes are assembled from profiled metal strips to constitute boundary walls of gas passages. Corona-discharge electrodes are centrally disposed in the gas passages and their tips extend in such directions that an optimum field for the collection of dust is established (see Published German Application No. 34 08 839).
The corona-discharge electrodes which are tensioned in a frame must be manufactured with great care to ensure that all strips or wires will be uniformly tensioned. If the electrodes are under an inadequate tension, the influences of the flowing gases and of the electric field forces will result in uncontrolled vibration and possibly in a decrease of the distance to the collecting electrodes so that undesired flashovers occur.
Excessively tensioned strips or wires may be torn off under the action of the rapping blows so that the dust-extracting capacity will be reduced and downtimes become necessary, which are expensive particularly in power plants. For this reason, special tools for uniformly tensioning the electrodes have been proposed (Published German Application No. 26 03 514).
Corona-discharge electrodes consisting of wires tensioned in frames can be made in a shop only up to certain sizes. Limits are imposed by the maximum dimensions which are permissible for transportation. But particularly in the design of power plants there is a trend toward larger units so that larger dust-collecting electrostatic precipitators are also required. For instance, a power unit of 740 megawatts requires two dust-collecting electrostatic precipitators each of which has a length of about 33 meters, a width of 37 meters and a height of 23 meters. Each precipitator has a projected collecting electrode surface area of about 70,000 m.sup.2 and contains corona electrodes having a total length of about 220 km (periodical "Technische Mitteilungen" (1978), No. 3, pages 123 to 131.
It will readily be understood that one-piece mounting frames cannot be used in conjunction with collecting electrodes having a height up to 15 meters. But even frames which have a height of 7.5 meters and have a width of about 6 meters in the direction of gas flow can no longer be transported. If they are made on the building site, the compliance with the manufacturing tolerances, which must be very small, involves a considerable expenditure. Besides, such large frames can be handled only with difficulty.
Some operators of dust-collecting electrostatic precipitators hesitate to use corona-discharge electrodes consisting of wires tensioned in frames. While such electrodes can be installed and aligned quickly, they do not permit of a repair of individual wires. For a repair, the entire frame must be removed and when a new wire has been fixed all wires must be retensioned or must at least be checked whether they are uniformly tensioned.
Other corona-discharge electrodes have been disclosed which consist of relatively rigid components, such as profiled plates or masts or the like (see U.S. Pat. Nos. 3,435,594 & 4,321,068). In such cases the design of the corona discharge electrodes will determine the required design of the collecting electrodes so that the collecting electrodes are designed in adaptation to the corona discharge electrodes rather than vice versa as is the case with wires tensioned in frames. This need not be a disadvantage. But because collecting electrodes having an optimum profiled shape have been developed for use with wires tensioned in frames, it is desired to use such collecting electrodes also where corona discharge electrodes consisting of wires tensioned in frames cannot be adopted.